FIG. 1 shows an axial view of a typical prior art third generation CT scanner 10 that includes an X-ray source 12 and an X-ray detector system 14 secured respectively to diametrically opposite sides of an annular-shaped disk 16. The disk is rotatably mounted within a gantry support (not shown) so that during a tomographic scan, the disk continuously rotates about a rotation axis 18 (which is normal to the plane of the page in FIG. 1) while X-rays pass from the source 12 through an object, such as a human patient 20 positioned within the opening of the disk, to the detector system 14.
The detector system 14 typically includes an array of individual detectors 22 disposed, for example, as a single row in the shape of an arc of a circle having a center of curvature at the point 24, referred to as the "focal spot", where the radiation emanates from the X-ray source 12. Other detector systems are known. The X-ray source and the array of detectors are positioned so that the X-ray paths between the source and each of the detectors all lie in the same plane (hereinafter the "rotation plane" or "scanning plane") which is normal to the rotation axis 18 of the disk. Since the X-ray paths originate from what is substantially a point source and extend at different angles to the detectors, the X-ray paths form a "fan beam" 26 that is incident on the detector system 14. The X-rays incident on a single detector at a measuring interval during a scan are commonly referred to as a "ray", and each detector generates an analog output signal indicative of the intensity of its corresponding ray. Since each ray is partially attenuated by all the mass in its path, the analog output signal generated by each detector is representative of an integral of the density of all the mass disposed between that detector and the X-ray source (i.e., the density of the mass lying in the detector's corresponding ray path) for that measuring interval.
The analog output signals generated by the X-ray detectors are normally processed by a signal processing portion (not shown in FIG. 1) of the CT system. The signal processing portion usually includes an analog low pass filter and a DAS. The low pass filter removes high frequency components from the analog output signals generated by the X-ray detectors, and the DAS filters the analog output signals generated by the low pass filter to improve their signal-to-noise ratio. One such DAS is described in U.S. Pat. No. 4,547,893, which is assigned to the assignee of the present invention. Other types of DASs are well known. The DAS also normally samples the filtered signals to generate digital output signals representative of the acquired projection data during each projection. The digital output signals generated by the DAS are referred to herein as "projection data signals". The collection of all the projection data signals at a measuring interval is commonly referred to as a "projection" or a "view", and the angular orientation of the X-ray source 12 and detector system 14 on the disk 16 corresponding to a particular projection or view is referred to as the "projection angle".
During a single tomographic scan, ideally the disk 16 rotates smoothly and continuously around the object being scanned allowing the scanner 10 to generate a plurality of projections precisely at a corresponding plurality of projection angles. In a typical tomographic scan, the disk rotates at least 360.degree. around the object being scanned, and the scanner ideally generates a new projection every time the disk rotates an equal incremental amount referred to as .DELTA..theta.. For example, where .DELTA..theta. is 0.125 degrees for a 360.degree. scan, the scanner generates 2,880 (i.e., eight times 360) projections. The sampling interval between measuring adjacent projections (i.e., the time required for the disk to rotate through an angle of .DELTA..theta.) is typically on the order of a millisecond.
Using well known algorithms, such as the inverse Radon transform, a tomogram may be generated from the data acquired at all the projections measured at the corresponding projection angles. A tomogram is representative of the density of a two dimensional "slice" of the object being scanned. The process of generating a tomogram from the projections is commonly referred to as "filtered back projection" or "reconstruction", since the tomogram may be thought of as being reconstructed from the projection data. The signal processing portion normally includes a back projector for generating the tomograms from the projections.
It is generally assumed that there are at least two requirements for acquiring data during a scan. First, the data must be acquired with the best possible dynamic range and the best possible signal-to-noise ratio. Secondly, the data must be taken at precise and known angles of rotation so that a simpler and faster reconstruction algorithm can be used. These reconstruction algorithms generally assume that the projection data used to reconstruct a single tomogram are generated at equally spaced projection angles (i.e., that the disk 16 rotates by exactly, no more and no less than, .DELTA..theta. between the generation of data for each successive projection). If the projections used to reconstruct a tomogram are not generated at equally spaced projection angles, the resulting tomogram will normally include unwanted artifacts. However, prior art CT scanners have had difficulty in generating projections precisely at equally spaced projection angles.
There are at least three sources of possible errors relating to the acquisition of data at precise and known angles of rotation. First, the rotational speed of the disk, may not be constant, and therefore making projection measurements at equal time intervals, will not necessarily result in projection data at precisely equal angular position intervals of .DELTA..theta.. Second, the measurement of the angles may be inaccurate. Third, the angle markers and position measurements may not be precise enough.
With regard to the first source of error, as stated above, during a typical tomographic scan, the disk 16 rotates at least 360.degree., and this rotation is normally accomplished in a time period on the order of two seconds. In practice it is extremely difficult to control the rotation of disk 16 so that the disk rotates with precise constant angular velocity. Rather, the rotational speed of disk 16 is typically characterized by some irregularity, or phase noise, sometimes referred to as jitter. FIG. 2 shows a graph that illustrates the effects of this rotational irregularity. The Y-axis of FIG. 2 represents the angular orientation of the disk 16 and the X-axis represents time during a tomographic scan. An ideal, linear, constant angular velocity trajectory for disk 16 is shown in FIG. 2 by Curve A, i.e., a straight line. Curve B in FIG. 2 illustrates an angular trajectory for disk 16 that is characterized by rotational irregularity. If disk 16 follows the ideal trajectory of Curve A, projections generated at constant rate or frequency (or at sampling times that are equally spaced apart in time) will be generated at equally spaced projection angles. As shown in FIG. 2, if three projections are generated at sampling times T.sub.1, T.sub.2, and T.sub.3, where these sampling times are separated by intervals of equal length .DELTA.t, the three projections will be equally separated in terms of their associated projection angles by equal angular intervals, each of .DELTA..theta.. However, projections generated at these sampling times when the disk follows the non-ideal trajectory of Curve B will not be equally separated in terms of their projection angle. As shown by the example shown in FIG. 2, projections generated at times T.sub.1 and T.sub.2 will have associated projection angles that are separated by an angular interval of (.DELTA..theta.-.delta..sub.1), whereas projections generated at times T.sub.2 and T.sub.3 will have associated projection angles separated by an angular interval of (.DELTA..theta.+.delta..sub.2). So the rotational irregularity of disk 16 causes projections generated at a constant rate or frequency to be unequally angularly spaced apart in terms of their associated projection angles. The magnitude of the rotational irregularity present in most, if not all, prior art CT scanners is normally sufficiently large to significantly degrade the quality of tomograms produced using projections generated at a constant rate or frequency.
FIG. 3 shows a block diagram of another prior art CT scanner 100 that provides compensation for the rotational irregularity of the disk. Scanner 100 includes a rotating disk 116 which rotates relative to a stationary gantry 130. A detector system 114, a low pass filter array 120, a DAS 132, and an X-ray source (such as source 12 which is shown in FIG. 1) are mounted on, and rotate with, disk 116. Stationary gantry 130 includes a computer or CT processor 138 for running the reconstruction algorithm with the raw data received from the disk. DAS 132 is illustrated as including a plurality or array 134 of analog-to-digital converters and a multiplexer 136. As is well known, the DAS may also include additional components such as the filters described in the above-referenced U.S. Pat. No. 4,547,893. Detector system 114 is an N-channel linear array including N individual detectors Di, for all i from one to N. Similarly, low pass filter array 120 and converter array 134 are N-channel arrays including N individual filters LPFi and analog-to-digital converters ADCi, respectively, for all i from one to N.
During a tomographic scan, while the disk 116 rotates through each of a plurality of projection angles, the detector array 114 generates N analog output signals that are applied to the low pass filter array 120. Specifically, the analog signal in the ith channel generated by the ith channel detector Di is applied to the ith channel low pass filter LPFi, for all i from one to N. The ith low pass filter LPFi receives the analog signal generated by detector Di and generates therefrom a filtered analog signal that is applied to the ith analog-to-digital converter ADCi, for all i from one to N. The ith analog-to-digital converter ADCi samples the filtered signal generated by the ith filter LPFi to generate a sampled raw data signal RawDi, for all i from one to N. The ith raw data signal RawDi includes a set of samples, or data points, RawDi(Tj) collected as sampling times Tj for all j from 1 to J, where J represents the total number of projections per scan. At any given sampling time Tj, the collection of the N data points RawDi(Tj), for all i from one to N, may be thought of as forming a single projection generated at a projection angle corresponding to the sampling time Tj. The N data points RawDi(Tj) in a projection are applied in time multiplexed fashion, via multiplexer 136, to the processor 138 on the stationary gantry 130. As is well known, multiplexer 136 is useful for reducing the number of connections between the rotating disk 116 and the stationary gantry 130. CT processor 138 generates tomograms from the projection data collected by rotating disk 116.
Also mounted in disk 116 is a system (not shown) for measuring the angular orientation of the disk in real time and for generating a Disk Rotation Angle signal that is representative of this angular orientation. Examples of prior art systems for generating the Disk Rotation Angle signal are disclosed in U.S. Pat. No. 5,432,339 issued Jul. 11, 1995 in the names of Bernie Gordon, David Winston, Paul Wagoner and Douglas Abraham and entitled Apparatus for and Method of Measuring Geometric, Positional and Kinematic Parameters of a Rotating Device; and pending U.S. patent application Ser. No. 08/948,493 filed Oct. 10, 1997 in the names of Geoffrey A. Legg, Gerard P. Riley, and Hans J. Weedon and entitled Measurement and Control System for Controlling System Functions as a Function of Rotational Parameters of a Rotating Device (Attorney's Docket No. ANA-139), both the prior patent and application being assigned to the present assignee. These systems sense markers as the disk rotates, and generate a rotation angle signal, similar to the Disk Rotation Angle signal mentioned above, when the relevant markers corresponding the projection angles are sensed. This Disk Rotation Angle signal is applied to the CT processor 138. The processor 138 monitors the orientation of disk 116 and generates a Variable Rate Sample Clock signal that is applied to and controls the operation of the analog-to-digital converter array 134. Processor 138 controls the analog-to-digital converter array 134 (via the Variable Rate Sample Clock signal) so that the converter array 134 generates projections (i.e., samples the analog filtered signals) at equally spaced projection angles. As illustrated by FIG. 2, when the disk rotation is characterized by irregularities, such equally spaced projections can not be generated by sampling at constant frequency. Rather, stationary gantry 130 continually adjusts the phase or frequency of the Variable Rate Sample Clock signal in response to detected disk rotational irregularity (as measured by the Disk Rotation Angle signal) so that the converter array 134 generates projections at the desired projection angles. Since the projections in scanner 100 are not generated at constant frequency (i.e., are not generated at equally spaced time intervals), scanner 100 is referred to as a "sample-on-demand" type scanner.
In principle, sample-on-demand scanners can provide effective compensation for disk rotational irregularities. However, there are several problems associated with sample-on-demand scanners of the type shown in FIG. 3. One problem relates to the interaction of the low pass filter array 120 and the analog-to-digital converter array 134. The low pass filters in array 120 are each characterized by a transfer function, and the analog-to-digital converters in array 134 are each characterized by another transfer function. The transfer function used to generate each raw data signal RawDi is essentially a combination of the corresponding low pass filter transfer function and analog-to-digital converter transfer function. The low pass filter transfer function is normally selected so that any signal components above a selected frequency f.sub.max in the detector output signals are suppressed in the filtered signals. The selected frequency f.sub.max is normally related to the sampling frequency used by the analog-to-digital converter array 134 to insure that high frequency components in the detector output signals are not aliased into the raw data signals.
The low pass filters in array 120 are normally implemented using analog RC (resistor-capacitor) networks, and the low pass filter transfer function of each filter is determined by the values of the components of the corresponding RC network. The analog-to-digital converter transfer function is in part a function of the phase and frequency of the Variable Rate Sample Clock signal. So, when a constant frequency, stable phase, Variable Rate Sample Clock signal is applied to analog-to-digital converter array 134, the same transfer function is used to generate every data point RawDi(Tj), for all channels i and for all sampling times j. However, adjusting the phase or frequency of the Variable Rate Sample Clock signal during a CT scan will prevent all of the data points RawDi(Tj) from being generated using the same transfer function. For example, the transfer function used to generate the data point RawD1(T5) may be different than the transfer function used to generate the later data point RawD1(T7). Ideally, when the phase or frequency of the Variable Rate Sample Clock signal is varied (and the analog-to-digital converter transfer function is correspondingly varied) a compensatory change should be introduced into the low pass filter transfer function to insure that every data point RawDi(Tj) (and every projection) is generated using the same combined transfer function. However, since the low pass filter transfer function is determined by the particular RC network used to implement the filters, there is no simple way to introduce the desired time varying changes to the low pass filter transfer function corresponding to variations in the phase or frequency of the Variable Rate Sample Clock signal.
So, while varying the phase and frequency of the Variable Rate Sample Clock signal is desirable to insure that all projections in a scan are generated at precisely equally spaced projection angle intervals, this variance of the Variable Rate Sample Clock signal prevents all projections from being generated using the same combined transfer function. This tends to increase artifacts and reduce the signal-to-noise ratio, in the resulting tomograms.
Another problem with sample-on-demand scanner of the type shown in FIG. 3 is that the analog-to-digital converters in array 134 do not operate at a constant frequency, but instead must operate at a variable phase or frequency under control of the Variable Rate Sample Clock signal. Such converters are more expensive to construct than constant frequency analog-to-digital converters. The cost and complexity of CT scanners could be reduced if the analog-to-digital converters could operate at a constant sampling frequency. However, this has not been considered possible in prior art scanners because of the problems associated with disk rotation irregularities.
These and other problems with, and limitations of, the prior art CT scanners are overcome with the CT scanner of this invention.